// Ami Bar
// amibar@gmail.com

using System;
using System.Threading;

namespace Amib.Threading.Internal
{

    #region WorkItemsQueue class

    /// <summary>
    /// WorkItemsQueue class.
    /// </summary>
    public class WorkItemsQueue : IDisposable
    {
        #region Member variables

        /// <summary>
        /// Each thread in the thread pool keeps its own waiter entry.
        /// </summary>
        [ThreadStatic] private static WaiterEntry _waiterEntry;

        /// <summary>
        /// Waiters queue (implemented as stack).
        /// </summary>
        private readonly WaiterEntry _headWaiterEntry = new WaiterEntry();

        /// <summary>
        /// Work items queue
        /// </summary>
        private readonly PriorityQueue _workItems = new PriorityQueue();

        /// <summary>
        /// A flag that indicates if the WorkItemsQueue has been disposed.
        /// </summary>
        private bool _isDisposed;

        /// <summary>
        /// Indicate that work items are allowed to be queued
        /// </summary>
        private bool _isWorkItemsQueueActive = true;

        /// <summary>
        /// Waiters count
        /// </summary>
        private int _waitersCount;

        #endregion

        #region Public properties

        /// <summary>
        /// Returns the current number of work items in the queue
        /// </summary>
        public int Count
        {
            get
            {
                lock (this)
                {
                    ValidateNotDisposed();
                    return _workItems.Count;
                }
            }
        }

        /// <summary>
        /// Returns the current number of waiters
        /// </summary>
        public int WaitersCount
        {
            get
            {
                lock (this)
                {
                    ValidateNotDisposed();
                    return _waitersCount;
                }
            }
        }

        #endregion

        #region Public methods

        /// <summary>
        /// Enqueue a work item to the queue.
        /// </summary>
        public bool EnqueueWorkItem(WorkItem workItem)
        {
            // A work item cannot be null, since null is used in the
            // WaitForWorkItem() method to indicate timeout or cancel
            if (null == workItem)
            {
                throw new ArgumentNullException("workItem", "workItem cannot be null");
            }

            bool enqueue = true;

            // First check if there is a waiter waiting for work item. During 
            // the check, timed out waiters are ignored. If there is no 
            // waiter then the work item is queued.
            lock (this)
            {
                ValidateNotDisposed();

                if (!_isWorkItemsQueueActive)
                {
                    return false;
                }

                while (_waitersCount > 0)
                {
                    // Dequeue a waiter.
                    WaiterEntry waiterEntry = PopWaiter();

                    // Signal the waiter. On success break the loop
                    if (waiterEntry.Signal(workItem))
                    {
                        enqueue = false;
                        break;
                    }
                }

                if (enqueue)
                {
                    // Enqueue the work item
                    _workItems.Enqueue(workItem);
                }
            }
            return true;
        }


        /// <summary>
        /// Waits for a work item or exits on timeout or cancel
        /// </summary>
        /// <param name="millisecondsTimeout">Timeout in milliseconds</param>
        /// <param name="cancelEvent">Cancel wait handle</param>
        /// <returns>Returns true if the resource was granted</returns>
        public WorkItem DequeueWorkItem(
            int millisecondsTimeout,
            WaitHandle cancelEvent)
        {
            /// This method cause the caller to wait for a work item.
            /// If there is at least one waiting work item then the 
            /// method returns immidiately with true.
            /// 
            /// If there are no waiting work items then the caller 
            /// is queued between other waiters for a work item to arrive.
            /// 
            /// If a work item didn't come within millisecondsTimeout or 
            /// the user canceled the wait by signaling the cancelEvent 
            /// then the method returns false to indicate that the caller 
            /// didn't get a work item.

            WaiterEntry waiterEntry = null;
            WorkItem workItem = null;

            lock (this)
            {
                ValidateNotDisposed();

                // If there are waiting work items then take one and return.
                if (_workItems.Count > 0)
                {
                    workItem = _workItems.Dequeue() as WorkItem;
                    return workItem;
                }
                    // No waiting work items ...
                else
                {
                    // Get the wait entry for the waiters queue
                    waiterEntry = GetThreadWaiterEntry();

                    // Put the waiter with the other waiters
                    PushWaiter(waiterEntry);
                }
            }

            // Prepare array of wait handle for the WaitHandle.WaitAny()
            var waitHandles = new[]
                                  {
                                      waiterEntry.WaitHandle,
                                      cancelEvent
                                  };

            // Wait for an available resource, cancel event, or timeout.

            // During the wait we are supposes to exit the synchronization 
            // domain. (Placing true as the third argument of the WaitAny())
            // It just doesn't work, I don't know why, so I have lock(this) 
            // statments insted of one.

            int index = WaitHandle.WaitAny(
                waitHandles,
                millisecondsTimeout,
                true);

            lock (this)
            {
                // success is true if it got a work item.
                bool success = (0 == index);

                // The timeout variable is used only for readability.
                // (We treat cancel as timeout)
                bool timeout = !success;

                // On timeout update the waiterEntry that it is timed out
                if (timeout)
                {
                    // The Timeout() fails if the waiter has already been signaled
                    timeout = waiterEntry.Timeout();

                    // On timeout remove the waiter from the queue.
                    // Note that the complexity is O(1).
                    if (timeout)
                    {
                        RemoveWaiter(waiterEntry, false);
                    }

                    // Again readability
                    success = !timeout;
                }

                // On success return the work item
                if (success)
                {
                    workItem = waiterEntry.WorkItem;

                    if (null == workItem)
                    {
                        workItem = _workItems.Dequeue() as WorkItem;
                    }
                }
            }
            // On failure return null.
            return workItem;
        }

        /// <summary>
        /// Cleanup the work items queue, hence no more work 
        /// items are allowed to be queue
        /// </summary>
        protected virtual void Cleanup()
        {
            lock (this)
            {
                // Deactivate only once
                if (!_isWorkItemsQueueActive)
                {
                    return;
                }

                // Don't queue more work items
                _isWorkItemsQueueActive = false;

                foreach (WorkItem workItem in _workItems)
                {
                    workItem.DisposeOfState();
                }

                // Clear the work items that are already queued
                _workItems.Clear();

                // Note: 
                // I don't iterate over the queue and dispose of work items's states, 
                // since if a work item has a state object that is still in use in the 
                // application then I must not dispose it.

                // Tell the waiters that they were timed out.
                // It won't signal them to exit, but to ignore their
                // next work item.
                while (_waitersCount > 0)
                {
                    WaiterEntry waiterEntry = PopWaiter();
                    waiterEntry.Timeout();
                }
            }
        }

        #endregion

        #region Private methods

        /// <summary>
        /// Returns the WaiterEntry of the current thread
        /// </summary>
        /// <returns></returns>
        /// In order to avoid creation and destuction of WaiterEntry
        /// objects each thread has its own WaiterEntry object.
        private WaiterEntry GetThreadWaiterEntry()
        {
            if (null == _waiterEntry)
            {
                _waiterEntry = new WaiterEntry();
            }
            _waiterEntry.Reset();
            return _waiterEntry;
        }

        #region Waiters stack methods

        /// <summary>
        /// Push a new waiter into the waiter's stack
        /// </summary>
        /// <param name="newWaiterEntry">A waiter to put in the stack</param>
        public void PushWaiter(WaiterEntry newWaiterEntry)
        {
            // Remove the waiter if it is already in the stack and 
            // update waiter's count as needed
            RemoveWaiter(newWaiterEntry, false);

            // If the stack is empty then newWaiterEntry is the new head of the stack 
            if (null == _headWaiterEntry._nextWaiterEntry)
            {
                _headWaiterEntry._nextWaiterEntry = newWaiterEntry;
                newWaiterEntry._prevWaiterEntry = _headWaiterEntry;
            }
                // If the stack is not empty then put newWaiterEntry as the new head 
                // of the stack.
            else
            {
                // Save the old first waiter entry
                WaiterEntry oldFirstWaiterEntry = _headWaiterEntry._nextWaiterEntry;

                // Update the links
                _headWaiterEntry._nextWaiterEntry = newWaiterEntry;
                newWaiterEntry._nextWaiterEntry = oldFirstWaiterEntry;
                newWaiterEntry._prevWaiterEntry = _headWaiterEntry;
                oldFirstWaiterEntry._prevWaiterEntry = newWaiterEntry;
            }

            // Increment the number of waiters
            ++_waitersCount;
        }

        /// <summary>
        /// Pop a waiter from the waiter's stack
        /// </summary>
        /// <returns>Returns the first waiter in the stack</returns>
        private WaiterEntry PopWaiter()
        {
            // Store the current stack head
            WaiterEntry oldFirstWaiterEntry = _headWaiterEntry._nextWaiterEntry;

            // Store the new stack head
            WaiterEntry newHeadWaiterEntry = oldFirstWaiterEntry._nextWaiterEntry;

            // Update the old stack head list links and decrement the number
            // waiters.
            RemoveWaiter(oldFirstWaiterEntry, true);

            // Update the new stack head
            _headWaiterEntry._nextWaiterEntry = newHeadWaiterEntry;
            if (null != newHeadWaiterEntry)
            {
                newHeadWaiterEntry._prevWaiterEntry = _headWaiterEntry;
            }

            // Return the old stack head
            return oldFirstWaiterEntry;
        }

        /// <summary>
        /// Remove a waiter from the stack
        /// </summary>
        /// <param name="waiterEntry">A waiter entry to remove</param>
        /// <param name="popDecrement">If true the waiter count is always decremented</param>
        private void RemoveWaiter(WaiterEntry waiterEntry, bool popDecrement)
        {
            // Store the prev entry in the list
            WaiterEntry prevWaiterEntry = waiterEntry._prevWaiterEntry;

            // Store the next entry in the list
            WaiterEntry nextWaiterEntry = waiterEntry._nextWaiterEntry;

            // A flag to indicate if we need to decrement the waiters count.
            // If we got here from PopWaiter then we must decrement.
            // If we got here from PushWaiter then we decrement only if
            // the waiter was already in the stack.
            bool decrementCounter = popDecrement;

            // Null the waiter's entry links
            waiterEntry._prevWaiterEntry = null;
            waiterEntry._nextWaiterEntry = null;

            // If the waiter entry had a prev link then update it.
            // It also means that the waiter is already in the list and we
            // need to decrement the waiters count.
            if (null != prevWaiterEntry)
            {
                prevWaiterEntry._nextWaiterEntry = nextWaiterEntry;
                decrementCounter = true;
            }

            // If the waiter entry had a next link then update it.
            // It also means that the waiter is already in the list and we
            // need to decrement the waiters count.
            if (null != nextWaiterEntry)
            {
                nextWaiterEntry._prevWaiterEntry = prevWaiterEntry;
                decrementCounter = true;
            }

            // Decrement the waiters count if needed
            if (decrementCounter)
            {
                --_waitersCount;
            }
        }

        #endregion

        #endregion

        #region WaiterEntry class 

        // A waiter entry in the _waiters queue.
        public class WaiterEntry : IDisposable
        {
            #region Member variables

            private bool _isDisposed;

            /// <summary>
            /// Flag to know if the waiter was signaled and got a work item. 
            /// </summary>
            private bool _isSignaled;

            /// <summary>
            /// Flag to know if this waiter already quited from the queue 
            /// because of a timeout.
            /// </summary>
            private bool _isTimedout;

            // Linked list members
            internal WaiterEntry _nextWaiterEntry;
            internal WaiterEntry _prevWaiterEntry;

            /// <summary>
            /// Event to signal the waiter that it got the work item.
            /// </summary>
            private AutoResetEvent _waitHandle = new AutoResetEvent(false);

            /// <summary>
            /// A work item that passed directly to the waiter withou going 
            /// through the queue
            /// </summary>
            private WorkItem _workItem;

            #endregion

            #region Construction

            public WaiterEntry()
            {
                Reset();
            }

            #endregion

            #region Public methods

            public WaitHandle WaitHandle
            {
                get { return _waitHandle; }
            }

            public WorkItem WorkItem
            {
                get
                {
                    lock (this)
                    {
                        return _workItem;
                    }
                }
            }

            /// <summary>
            /// Signal the waiter that it got a work item.
            /// </summary>
            /// <returns>Return true on success</returns>
            /// The method fails if Timeout() preceded its call
            public bool Signal(WorkItem workItem)
            {
                lock (this)
                {
                    if (!_isTimedout)
                    {
                        _workItem = workItem;
                        _isSignaled = true;
                        _waitHandle.Set();
                        return true;
                    }
                }
                return false;
            }

            /// <summary>
            /// Mark the wait entry that it has been timed out
            /// </summary>
            /// <returns>Return true on success</returns>
            /// The method fails if Signal() preceded its call
            public bool Timeout()
            {
                lock (this)
                {
                    // Time out can happen only if the waiter wasn't marked as
                    // signaled
                    if (!_isSignaled)
                    {
                        // We don't remove the waiter from the queue, the DequeueWorkItem 
                        // method skips _waiters that were timed out.
                        _isTimedout = true;
                        return true;
                    }
                }
                return false;
            }

            /// <summary>
            /// Reset the wait entry so it can be used again
            /// </summary>
            public void Reset()
            {
                _workItem = null;
                _isTimedout = false;
                _isSignaled = false;
                _waitHandle.Reset();
            }

            /// <summary>
            /// Free resources
            /// </summary>
            public void Close()
            {
                if (null != _waitHandle)
                {
                    _waitHandle.Close();
                    _waitHandle = null;
                }
            }

            #endregion

            #region IDisposable Members

            public void Dispose()
            {
                if (!_isDisposed)
                {
                    Close();
                    _isDisposed = true;
                }
            }

            #endregion

            ~WaiterEntry()
            {
                Dispose();
            }
        }

        #endregion

        #region IDisposable Members

        public void Dispose()
        {
            if (!_isDisposed)
            {
                Cleanup();
                _isDisposed = true;
                GC.SuppressFinalize(this);
            }
        }

        #endregion

        ~WorkItemsQueue()
        {
            Cleanup();
        }

        private void ValidateNotDisposed()
        {
            if (_isDisposed)
            {
                throw new ObjectDisposedException(GetType().ToString(), "The SmartThreadPool has been shutdown");
            }
        }
    }

    #endregion
}